T cell responses are interesting. Infectolab (https://www.infectolab-americas.com/) and its originator Armin (Germany) are using T cell responses for tick borne diseases like: Borellia (Lyme), Bartonella and Babesia because antibody testing is difficult for these (broken via CDC specs as most labs won't test key proteins 31, 34 and never indicate which antibodies are present). Bartonella is very hard to test for with antibody and PCR. It really depends on lab specialty. The sars-cov-2 antibody tests are unpredictable in quality, so nearly useless (my dr is 0-50 via Quest and many people had + PCR).
In short, our immune context (genetic phenotype) is unique! We need a lot more data from everyone to start making accurate correlations. We do not measure T-cells, cytokines, mast cells, b-cells, HLA (partly how we potentially make antibodies) at any meaningful level to provide much confidence. Many natural/industrial substances suppress our T-cell responses and generally innate immune system (metals, mold toxins, etc), so we also need to start accounting for those.
It's a long road we have in front of us. Hopefully the medical system supports patient data ownership and research to improve on our obvious ignorance.